1. Field of the Invention
The present invention relates to an ink set for ink jet recording, the ink set including a black ink and a plurality of color inks, and to an ink jet recording method.
2. Description of the Related Art
One of the methods for recording characters and images known in the art is an ink jet recording method in which characters and images are formed on a recording medium with inks ejected onto the recording medium through nozzles of a recording head. In order to improve the print quality in the ink jet recording method, various techniques have been employed. One of such techniques is to use inks having permeability suitable for ink jet recording.
For example, there is known an ink jet recording method in which inks that have a low permeation rate on recording paper, i.e., a recording medium, are used so that the amount of ink adhering on the surface and the nearby part of the surface of the recording medium is large, and so that the density and sharpness of the characters and drawings can be improved. An ink that has a low permeation rate and mostly remains on the surface of the recording medium is generally called “low-permeability ink,” whereas an ink that has a high permeation rate is called “super-permeability ink.”
As illustrated in FIG. 5A, in the cases where a super-permeability ink having high permeability is used, an ink 51 permeates into the interior of a recording medium 52 immediately after the ink 51 is ejected onto the recording medium 52, and the amount of the ink 51 remaining on the surface is small. The rate of permeation of the ink 51 into the recording medium 52 is high, and the ink 51 may reach as far as the rear surface of the recording medium 52, depending on the permeability and other characteristics of the material of the recording medium 52.
An ink that quickly permeates into a recording medium as soon as the ink lands on the recording medium rarely mixes with other inks on the recording medium. Thus, such an ink is advantageous in that bleeding at the boundary regions of different colors can be suppressed. However, since the ink deeply permeates into the recording medium and diffuses over a wide range, coloring components of the dye or pigment become diffused. Moreover, since light that is incident on the recording medium is reflected at a deep position with respect to the surface of the recording medium, the density of the recorded image tends to be low. Furthermore, the ink 51 may diffuse widely in all directions, resulting in an excessive increase in size of the recording dot, and whisker-like bleeding (feathering) may occur around the edge of the dot, resulting in an image with a blurred contour.
In contrast, as illustrated in FIG. 5B, when a low-permeability ink 53 having low permeability is used, the low-permeability ink 53 remains protruding on the surface of the recording medium 52, and the components of the ink such as a solvent can easily evaporate under such a condition. Thus, the amount of low-permeability ink 53 permeating into the recording medium 52 in the thickness direction is small.
When the amount of ink remaining on the recording medium surface is large, the density of the image tends to be high. Moreover, since the amount of ink diffusing into the recording medium in one dot is significantly lower than in the case of a super-permeability ink, a low-permeability ink can record sharp images. However, the rate of permeation of this type of ink into the recording medium is low, and the time required for the ink on the recording medium surface to fix is long. Thus, when other inks are ejected onto nearby positions, flow-out of inks occurs between the inks. Thus, bleeding occurs in the boundary portion between different colors, and the image quality tends to be low.
It has been a wide practice up to the present to use a poorly permeable ink as a black ink and highly permeable inks as inks of other colors. A black ink is usually used for recording letters and drawings, minute lines and points of which must be clearly drawn and which must be easily recognizable. Thus, a low-permeability ink that can achieve high recording density and can record sharp contours has been used as a black ink. In recording a color image in which dots of different colors are usually disposed next to one another, super-permeability inks that can record sharp boundaries and that can suppress bleeding in the boundary regions between different colors have been used.
However, although different inks with different permeability have been used as a black ink and color inks to comply with different requirements for characteristics of images to be recorded, the following problems have occurred. As shown in FIG. 5C, when a black dot 54 of an ink having low permeability is adjacent to a color dot 55 of an ink having high permeability, flow-out may occur between the adjacent dots, thereby degrading the recording quality. In the black dot 54, the ink protruding from the recording medium surface flows out from a boundary portion 56 into the color dot 55, resulting in a decrease in density in the boundary portion 56 at the black dot side. As a result, the density of the contour of the black dot decreases, and a whitish blurred image is produced (undertrapping). In the color dot, the black ink mixes in the boundary portion 56, and the contour becomes unclear (bleeding). This case illustrates that when inks with different permeabilities are adjacent to each other, undertrapping or bleeding may occur in the boundary portion 56, and the quality of recording tends to be low. These phenomena are particularly frequent when recording of images is conducted at a high speed.
Many methods for preventing bleeding have been proposed. For example, Japanese Patent Laid-Open No. 60-197778 discloses a method in which inks having substantially the same surface tension (30 dyn/cm (mN/m) or more and 60 dyn/cm (mN/m) or less at 20° C.) are used as the color inks for forming color images. However, it has been difficult to obtain an image with sufficient optical density by using the inks, particularly the black ink, disclosed in Japanese Patent Laid-Open No. 60-197778. Moreover, since bleeding cannot be sufficiently prevented, the quality of the image is poor. Another concern is that there are many types of standard recording paper depending on region and manufacturer. It is desirable for the density of the black image and the sharpness at the edge portions to be high regardless of the type of paper that is used.
Japanese Patent Laid-Open No. 2000-198955 discloses a technique for improving the quality of the image formed by a black ink and preventing bleeding between a color image and a black image by using a salt-containing, self-dispersion pigment ink as the black ink. However, since ever higher speed and higher image quality are in demand, it is desirable to suppress bleeding at a higher level even when the time difference in printing between the black ink and other color inks is small.
Another available technique involves leaving the recording medium and the black ink ejected on the medium to stand for a time long enough to allow the ink, particularly a poorly permeable ink, to fix on the recording medium to an extent sufficient to prevent bleeding. However, according to this technique, a significantly large time difference must be provided between printing with the black ink and printing with the color inks. The technique thus has a low throughput and is not suitable for high-speed recording.
Another proposed technique involves decreasing (reduction of) the amount of ink supplied in one scanning operation and repeating scanning at predetermined time intervals (multipass printing). However, in multipass printing, the number of times of scanning operations is increased and the printing time per sheet (throughput) can be very long if the scanning intervals are large. Another concern is that a fixing unit including a heater for forcibly evaporating the ink is sometimes used. If the fixing unit is to be included, the size of the apparatus increases, and there is also a disadvantage of uneconomical energy consumption.